The invention relates generally to electronic circuits and, more particularly, to voltage reference circuits.
Integrated circuits, and other electronic circuits, often require operating voltages that are stable over process, voltage, and temperature variations. One type of circuit that is commonly used to provide stable voltages is the bandgap reference circuit. A bandgap reference circuit takes advantage of the unique characteristics of the bandgap energy of a semiconductor material (e.g., silicon) to provide a stable reference voltage. At a temperature of absolute zero (i.e., zero Kelvin), the bandgap energy of a semiconductor material is typically a physical constant. As the temperature of the semiconductor material rises from absolute zero, the bandgap energy of the material decreases (i.e., a negative temperature coefficient is displayed). The voltage across a forward biased PN junction (i.e., the junction between a positive (P) doped portion and a negative (N) doped portion of a semiconductor material) is an accurate indicator of the bandgap energy of a material. For this reason, the voltage across a forward biased PN junction will decrease as the temperature of the semiconductor material is raised. The rate at which the voltage decreases depends upon the junction (cross-sectional) area of the particular PN junction (as well as the semiconductor material being used). Therefore, the voltages across two forward biased PN junctions having different cross-sectional areas (but using the same semiconductor material) will vary at different rates with temperature, but each of these voltages can be traced back to the same bandgap voltage constant at absolute zero. The conventional bandgap reference circuit utilizes the voltage relationships between two forward biased PN junctions having different cross-sectional areas to achieve a relatively temperature insensitive output voltage.
In a conventional bandgap reference circuit, a feedback loop is used in conjunction with an operational amplifier to generate the reference voltage. The circuit basically operates as a feedback control loop to maintain the two input nodes of the operational amplifier at approximately the same potential in the steady state. A first input node (e.g., the non-inverting input node) of the operational amplifier is coupled to ground through a first PN junction (e.g., a diode or transistor). A second input node (e.g., the inverting input node) of the operational amplifier is coupled to ground through a resistor (R1) and a second PN junction that has a different cross-sectional area (typically larger) than the first PN junction. Substantially equal currents are forced through the first and second PN junctions during circuit operation. By carefully selecting circuit component values for the bandgap reference circuit, a system can be achieved that balances the negative temperature coefficient associated with one of the PN junctions with a positive temperature coefficient associated with the feedback loop to generate a relatively temperature insensitive output voltage.
Ideally, an operational amplifier will generate a zero output voltage when equal voltage levels are applied to the inverting and non-inverting inputs of the amplifier. In practice, however, a zero differential input voltage will generate a non-zero output He .voltage in an operational amplifier due to, among other things, asymmetries within the circuitry. For this reason, a small offset voltage (VOS) is typically defined for an operational amplifier that will result in an output voltage of zero when a zero differential input voltage is applied to the amplifier. The offset voltage associated with a particular operational amplifier can vary with operating temperature and drift over time. As can be appreciated, these changes in the offset voltage can introduce error into a bandgap reference circuit using the operational amplifier. In addition, operational amplifiers also typically suffer from a noise component known as 1/f noise that increases with decreasing frequency. This form of noise can also introduce error into a bandgap reference circuit using the operational amplifier.